Electrodeposition baths containing a mixture of boron-containing compounds and chlorhexidine

ABSTRACT

Disclosed is an electrodeposition bath comprising a mixture of (i) at least one boron-containing compounds and (ii) chlorhexidine for controlling the growth of microorganisms in the electrodeposition bath. The combination of (i) and (ii) at a low concentration provides better control of microbes than does either (i) or (ii) at higher concentrations.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit U.S. Provisional Application Ser. No. 60/934389 filed on Jun. 12, 2007, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an improved electrodeposition process. More particularly the present invention relates to an improved electrodeposition bath comprising a mixture of at least one boron-containing compound and chlorhexidine.

BACKGROUND OF THE INVENTION

Electrodeposition as a coating method has become increasingly important in the coatings industry. Globally, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.

As compared with non-electrophoretic coating means, electrodeposition offers the advantages of increased paint utilization, improved corrosion protection and relatively low environmental contamination. Electrodeposition typically offers environmental advantages because (1) electrodepositable coating compositions contain very little organic solvent and, (2) downstream processes, such as closed loop rinsing, can minimize loss of coating components to the surrounding environment during coating application.

The electrodeposition process is well known, and involves immersing an electroconductive substrate (that is, the work-piece) into a bath of an aqueous electrocoating composition. In the case of a cationic electrocoat composition, the work-piece serves as the cathode. After electrodeposition of a coating onto a workpiece, the electrocoated substrate is rinsed with an aqueous rinsing composition.

Typical rinsing operations have multiple stages which can include closed loop spray and/or dip applications. Such rinsing processes are well known in electrocoating processes, but for clarity a closed loop spray process removes excess electrocoat material from the substrate by spray washing the surface with deionized or reverse osmosis water. A dip application removes excess electrocoat material from a substrate by submerging the substrate in a tank of dionized or reverse osmosis water. The rinse composition can be re-circulated and re-used. In a typical electrocoat operation, the electrodeposition bath is ultrafiltered to remove ionic contaminants and the ultrafiltrate is used in the rinsing operations.

Recirculating the coating or rinse compositions is both economically and environmentally desirable. However, recirculation an aqueous coating or rinse composition can create an environment conducive to the growth of microorganisms such as algae, fungi and bacteria. Microorganisms can adversely affect the quality and appearance of an electrodeposited coating. Further, the presence of microorganisms in the electrocoating or rinsing composition can cause the formation of precipitates in the tanks, and variation in process parameters, for example, pH, conductivity, film build, throwpower (that is, the rate of film deposition relative to the position of the anode) and stability. Also, particulate “dirt” deposition and bio-fouling can occur, thereby detrimentally affecting the appearance of the applied coating and reducing system performance.

In early electrodeposition processes, the “ultrafiltrate” used in the rinse stages typically contained solvents, heavy metals, and other organic materials which assisted in the suppression of the aforementioned microorganism growth. However, as environmentally undesirable components such as volatile organic compounds (VOC), hazardous air pollutants (HAPs), and heavy metals, such as lead and chrome have been reduced, increased bacterial infestation has occurred.

A number of compounds for controlling the growth of bacteria in heavy metal-free, low organic solvent content-electrodeposition baths are known. For example, silver ion has been utilized, as well as oxidizing agents such as hydrogen peroxide and calcium hypochlorite. However, silver ion is costly and can contribute to dirt formation the electrodeposition bath. Oxidizing agents can oxidize organic components of the electrodepositable composition.

A microbiocide composition containing a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one can cause a rougher appearance than a coating composition without this microbiocide. Moreover, such microbiocide compositions can contain metal salts, for example, magnesium nitrate and magnesium chloride, which can cause coating defects due to gas generation at the cathode. Use of microbiocides may not be convenient, and they can lose their effectiveness over time. Moreover, some microbiocides can require special handling and disposal.

Halonitroalkanes can negatively affect the appearance of an applied coating and can contribute to corrosion of metallic parts.

U.S. Pat. No. 4,732,905 discloses a composition used to control microorganism growth in water systems. U.S. Pat. No. 6,017,431 discloses the use of sulfamic acid in electrodeposition baths. U.S. Pat. Nos. 3,937,679; 3,959,106; 3,975,346; and 4,001,101 disclose the use of boric acid as a solubilizing agent for ionic group-containing film-forming resins having onium salt groups, such as quaternary ammonium groups and ternary sulfonium groups. U.S. Pat. No. 4,443,569 discloses cathodically electrodepositable compositions based on a nitrogen base-containing binder containing tertiary amino groups and primary and/or secondary hydroxyl groups, and a metal compound. US2003/0033248 discloses an improved electrodeposition bath containing boric acid.

In view of the foregoing, a need exists for a heavy metal-free, low or no VOC electrodeposition bath that is resistant to biodegradation, while maintaining excellent coating application conditions, coating appearance and performance properties. The elimination of the necessity to handle toxic microbiocides that often are used in electrodeposition baths neutralized with organic acids is also desirable.

SUMMARY OF THE INVENTION

The invention provides an improved electrodeposition bath for microorganism resistance. The improvement comprises the inclusion of both chlorhexidine and an effective amount of a boron-containing compound selected from at least one of boric acid, boric acid equivalents, and mixtures thereof in the electrodeposition bath in an amount sufficient to retard the growth of microorganisms in the electrodeposition bath relative to their growth in the absence of said components. The electrodeposition bath comprises an aqueous dispersion of an aqueous carrier and a film forming binder. The film forming binder comprises an epoxy-amine adduct and blocked isocyanates.

DETAILED DESCRIPTION OF THE INVENTION

Other than in the operating examples or where otherwise indicated, numerical range limits or numerical parameters set forth herein are approximations. Range limitations used in the description of the invention and/or in the claims should be interpreted as if modified by the term “about”. Slight variances above or below stated range limitations are not necessarily outside of the intended scope of operation of the present invention. Ranges provided herein are continuous, and understood to incorporate any whole or fractional value within stated range limits, unless said value is specifically or specially excluded. As such, any value within a specified range may be relied upon as if that value is individually and specifically set out herein.

It has been found that incorporating a mixture of chlorhexidine and boron-containing compounds into an electrodeposition bath provides a level of microbe protection that is superior to using either compound individually at higher concentrations. Such results are unexpected, and it is further surprising that the use of boron-containing compounds and chlorhexidine in an effective amount to reduce microorganism growth in electrodeposition baths described herein can be accomplished without detriment to critical process parameters such as pH and conductivity of the bath.

Suitable boron-containing compounds include those selected from boric acid, boric acid equivalents, and mixtures thereof. As used herein and in the claims, by “boric acid equivalents” is meant any of the numerous boron-containing compounds that can hydrolyze in aqueous media to form boric acid. Specific, but non-limiting examples of boric acid equivalents include boron oxides, for example, B₂O₃; boric acid esters such as those obtained by the reaction of boric acid with an alcohol or phenol, for example, trimethyl borate, triethyl borate, tri-n-propyl borate, tri-n-butyl borate, triphenyl borate, triisopropyl borate, tri-t-amyl borate, tri-2-cyclohexylcyclohexyl borate, triethanolamine borate, triisopropylamine borate, and triisopropanolamine borate. Additionally, amino-containing borates and tertiary amine salts of boric acid may be useful. Such boron-containing compounds include, but are not limited to, 2-(beta-dimethylaminoisopropoxy)-4,5-dimethyl-1,3,2-d-ioxaborolane, 2-(beta-diethylaminoethoxy)-4,4,6-trimethyl-1,3,2-dioxaborin-ane, 2-(beta-dimethylaminoethoxy)-4,4,6-trimethyl-1,3,2-dioxaborinane, 2-(betha-diisopropylaminoethoxy-1,3,2-dioxaborinane, 2-(beta-dibutylaminoethoxy)-4-m46hyl-1,3,2-dioxaborinane, 2-(gamma-dimethylaminopropoxy)-1,3,6,9-tetrapxa-2-boracycloundecane, and 2-(beta-dimethylaminoethoxy)-4,4-(4-hydorxybutyl)-1,3,2-dioxaborolane. Boric acid equivalents can also include metal salts of boric acid (i.e., metal borates) provided that such metal borates can readily dissociate in aqueous media to form boric acid. Suitable examples of metal borates useful in the electrodeposition bath of the present invention include, for example, calcium borate, potassium borates such as potassium metaborate, potassium tetraborate, potassium pentaborate, potassium hexaborate, and potassium octaborate, sodium borates such as sodium metaborate, sodium diborate, sodium tetraborate, sodium pentaborate, sodium perborate, sodium hexaborate, and sodium octaborate. Likewise, ammonium borates can be useful. Moreover, optional boron-containing compounds can be included, for example, bismuth borate and yttrium borate.

Suitable boric acid equivalents can also include organic oligomeric and polymeric compounds comprising boron-containing moieties. Suitable examples include polymeric borate esters, such as those formed by reacting an active hydrogen-containing polymer, for example, a hydroxyl functional group-containing acrylic polymer or polysiloxane polymer, with boric acid and/or a borate ester to form a polymer having borate ester groups.

Polymers suitable for this purpose can include any of a variety of active hydrogen-containing polymers such as those selected from at least one of acrylic polymers, polyepoxide polymers, polyester polymers, polyurethane polymers, polyether polymers and silicon-based polymers. By “silicon-based polymers” is meant a polymer comprising one or more —SiO— units in the backbone. Such silicon-based polymers can include hybrid polymers, such as those comprising organic polymeric blocks with one or more —SiO— units in the backbone. Preferably, boric acid is used in the electrodeposition bath of the present invention.

Boric acid or boric acid equivalents of the present invention are present in the electrocoat bath at a level ranging from greater than 0.3% to less than 2.0% of the total weight of the bath. Preferably, boric acid or boric acid equivalents of the present invention are present in the electrocoat bath at a level ranging from 0.4% to 1.7% of the total weight of the bath. Most preferably, boric acid or boric acid equivalents of the present invention are present in the electrocoat bath at a level ranging from 0.5% to 1.6% of the total weight of the bath.

Chlorhexidine is a known antiseptic compound. It is also known as 1,6-bis [5-(p-chlorophenyl)biguanidino]hexane and has the structural formula as shown in Figure I.

Chlorhexidine is present in the electrocoat bath at a level of from greater than 0.01% to 0.2% of the total weight of the bath. Preferably, chlorhexidine is present in the electrocoat bath at a level ranging from 0.02% to 0.18% of the total weight of the bath. Most preferably, chlorhexidine is present in the electrocoat bath at a level ranging from 0.03% to 0.16% of the total weight of the bath.

When chlorhexidine is present at its lowest level, 0.01% of the total weight of the bath, it is desired to keep the level of boric acid at greater than 0.5% of the total weight of the bath, more preferably at a level of greater than or equal to 1.0% of the total weight of the bath.

In another embodiment, the present invention is an electrodepositable composition suitable for use as an electrodeposition bath comprising film-forming resins having ionic salt groups wherein the electrodeposition bath includes boric acid and chlorhexidine. Examples of such film-forming resins are epoxy-based resins having amine salt groups and/or sulfonium salt groups.

In a preferred embodiment, the electrodepositable composition has a pH of 7 or less. At a pH of greater than 7, such cationic compositions tend to adsorb carbon dioxide from the surrounding atmosphere and, consequently, can drift below pH 7 over time. Therefore, compositions having a pH of 7 or less are more stable and process conditions are easier to control.

The term “principal emulsion” as used herein means an electrocoating composition comprising an aqueous emulsion of a binder of an epoxy amine adduct blended with a crosslinking agent which has been neutralized with an acid to form a water-soluble product. The binder of the electrocoating composition typically is a blend of an epoxy amine adduct and a blocked polyisocyanate crosslinking agent. While the microbiocides are potentially usable with a variety of different cathodic electrocoat resins, the epoxy amine adduct resins are particularly preferred. These resins are generally disclosed in U.S. Pat. No. 4,419,467 which is incorporated by reference.

Preferred crosslinkers for the epoxy amine adduct resins are also well known in the prior art. These are aliphatic, cycloaliphatic and aromatic isocyanates such as hexamethylene diisocyanate, cyclohexamethylene diisocyanate, toluene diisocyanate, methylene diphenyl diisocyanate and the like. These isocyanates are pre-reacted with a blocking agent such as oximes, alcohols, or caprolactams which block the isocyanate functionality, i.e., the crosslinking functionality. Upon heating the blocking agents separate, thereby providing a reactive isocyanate group and crosslinking occurs. Isocyanate crosslinkers and blocking agents are well known in the prior art and also are disclosed in the aforementioned U.S. Pat. No. 4,419,467.

The cathodic binder of the epoxy amine adduct and the blocked isocyanate are the principal resinous ingredients in the electrocoating composition and are usually present in amounts of about 30 to 50% by weight of solids of the composition. To form an electrocoating bath, the solids are generally reduced with an aqueous medium.

Besides the binder resin described above, the electrocoating composition usually contains pigment which is incorporated into the composition in the form of a pigment paste. The pigment paste is prepared by grinding or dispersing a pigment into a grinding vehicle and optional ingredients such as wetting agents, surfactants, and defoamers. Any of the pigment grinding vehicles that are well known in the art can be used or the novel additive described above can be used. After grinding, the particle size of the pigment should be as small as practical; generally, the particle size is about 6-8 using a Hegman grinding gauge.

Pigments which can be used in this invention include titanium dioxide, basic lead silicate, strontium chromate, carbon black, iron oxide, clay and the like. Pigments with high surface areas and oil absorbencies should be used judiciously because these can have an undesirable affect on coalescence and flow of the electrodeposited coating.

The pigment to binder weight ratio is also important and should be preferably less than 0.5:1, more preferably less than 0.4:1, and usually about 0.2:1 to 0.4:1. Higher pigment to binder weight ratios have been found to adversely affect coalescence and flow.

The coating compositions of the invention can contain optional ingredients such as wetting agents, surfactants, defoamers and the like. Examples of surfactants and wetting agents include alkyl imidazolines such as those available from Ciba-Geigy Industrial Chemicals, Tarrytown, N.Y., as “Amine C”, acetylenic alcohols available from Air Products and Chemicals, Allentown, Pa., as “Surfynol® 104”. These optional ingredients, when present, constitute from about 0.1 to 20 percent by weight of binder solids of the composition.

Optionally, plasticizers can be used to promote flow. Examples of useful plasticizers are high boiling water immiscible materials such as ethylene or propylene oxide adducts of nonyl phenols or bisphenol A. Plasticizers are usually used at levels of about 0.1 to 15 percent by weight resin solids.

The electrocoating composition of this invention is an aqueous dispersion. The term “dispersion” as used within the context of this invention is believed to be a two-phase translucent or opaque aqueous resinous binder system in which the binder is in the dispersed phase and water the continuous phase. The average particle size diameter of the binder phase is about 0.1 to 10 microns, preferably, less than 5 microns. The concentration of the binder in the aqueous medium in general is not critical, but ordinarily the major portion of the aqueous dispersion is water. The aqueous dispersion usually contains from about 3 to 50 percent preferably 5 to 40 percent by weight binder solids. Aqueous binder concentrates which are to be further diluted with water when added to an electrocoating bath generally have a range of binder solids of 10 to 30 percent weight.

EXAMPLES

Preparation of antimicrobial additive containing electrocoat samples. Amounts of boric acid and/or chlorhexidine were added to 49 ml samples of CORMAX® VI, available from DuPont, Wilmington, Del., according to Table 1. The amounts of boric acid and chlorhexidine listed in Table 1 are in percent by weight based on the total weight of the sample.

To evaluate the antimicrobial activity of the compounds, a “Challenge Inoculum” was prepared by isolating eleven unique bacteria from contaminated electrocoat baths at five different automotive assembly sites. Electrocoat samples were prepared by adding 1 milliliter (ml) of the challenge inoculum to 49 ml of electrocoat sample dispersion. This bacterial inoculation produced a bacterial count ranging from 1.0×10⁵ to 1.7×10⁶ CFU/ml. These challenged samples were incubated at room temperature with stirring and sterile air agitation (air at <0.1 Liters/minute) for 30 days. Samples were tested for the presence of bacteria by a standard plat count method after 1 hour, 24 hours, 1 week, 2 weeks, 3 weeks, and 30 days after inoculation. Tryptic Soy Agar (TSA) was used for enumeration of bacteria from the electrocoat samples using standard spread plate technique.

Table 1 shows the Bacterial Count data for the microbiocides tested and the control data.

TABLE 1 Challenge Sample Innoculum 1 Hour 24 Hours 1 Week 2 Weeks 3 Weeks 30 days Example Description (CFU/ml) (CFU/ml) (CFU/ml) (CFU/ml) (CFU/ml) (CFU/ml) (CFU/m) A CorMax ® VI 0.00 N.D. N.D. N.D. N.D. N.D. N.D. Control B CorMax ® VI 1.70 × 10⁶ 9.60 × 10⁴ 4.58 × 10⁵ 5.60 × 10⁶ 1.80 × 10⁷ 1.80 × 10⁷ 2.70 × 10⁷ Control C Boric Acid 1.50 × 10⁶ 1.45 × 10⁶ 5.35 × 10⁶ 1.90 × 10⁶ 3.36 × 10⁷ 4.75 × 10⁶ 1.28 × 10⁵ (1.0%) D Chlorhexidine 1.70 × 10⁶ 3.85 × 10⁶ 2.85 × 10⁶ 4.15 × 10⁷ 4.92 × 10⁷ 2.80 × 10⁸ 2.80 × 10⁸ (0.01%) E Boric Acid 1.30 × 10⁶ 1.74 × 10⁷ 2.05 × 10⁷ 5.70 × 10⁶ 1.50 × 10⁵ N.D. N.D. (1.5%) F Chlorhexidine 1.30 × 10⁶ 1.10 × 10³ 3.45 × 10² N.D. 2.25 × 10⁷ 8.00 × 10⁵ 2.50 × 10⁵ (0.1%) G Boric Acid 1.70 × 10⁶ 1.70 × 10⁶ 1.65 × 10⁶ 4.50 × 10⁴ 1.26 × 10³ N.D. N.D. (1.0%) + Chlorhexidine (0.01%) H Boric Acid 1.70 × 10⁶ 1.25 × 10⁶ 9.05 × 10⁴ 6.85 × 10² N.D. N.D. N.D. (1.0%) + Chlorhexidine (0.1%) I Boric Acid 1.00 × 10⁵ 1.75 × 10² 3.50 × 10¹ 2.20 × 10⁵ 2.21 × 10⁷ 4.91 × 10⁷ 1.40 × 10⁷ (0.5%) + Chlorhexidine (0.01%) J Boric Acid 1.00 × 10⁵ 1.00 × 10² 4.50 × 10¹ N.D. 9.65 × 10² 3.85 × 10⁵ N.D. (0.5%) + Chlorhexidine (0.05%) K Boric Acid 1.00 × 10⁵ 2.00 × 10¹ 1.50 × 10¹ N.D. N.D. N.D. N.D. (0.5%) + Chlorhexidine (0.1%) N.D. - No bacteria detected

The results of Table 1 show that the electrocoat baths containing only boric acid at 1.0% and 1.5% or chlorhexidine at 0.01% or 0.1% fail to give adequate control of the microbe population during the test period. Example E, containing 1.5% boric acid showed an increase in the microbe count before eventually controlling the population after 3 weeks. In contrast, electrocoat compositions containing 0.5% or greater of boric acid and greater than 0.01% chlorhexidine showed decreasing microbe populations.

Each of the electrocoat bath compositions above was tested to determine the effect of the biocide additive on the electrocoat bath conductivity and film build. The results of the tests are shown in Table 2.

TABLE 2 Conditions @ 25° F. Film Build in mils Conductivity @ 90° F. Bath Temperature Example Biocide pH μS/cm 175 V 200 V 225 V 250 V 275 V 300 V A CorMax ® VI 6.09 1882 0.705 0.801 0.913 1.036 1.178 1.292 (control) C Boric Acid (1.0%) 6.08 2069 0.653 0.768 0.906 1.048 1.301 rupture D Chlorhexidine 6.05 2245 0.619 0.693 0.793 0.915 1.020 1.170 (0.01%) E Boric Acid (1.5%) 5.71 1997 0.647 0.770 0.878 1.013 1.197 1.448 F Chlorhexidine 6.24 2222 0.658 0.729 0.811 0.888 0.999 1.150 (0.1%) G Boric Acid (1.0%) + 5.95 2008 0.672 0.738 0.861 0.984 1.172 1.381 Chlorhexidine (0.01%) H Boric Acid (1.0%) + 6.08 2059 0.704 0.872 0.918 1.137 1.366 1.444 Chlorhexidine (0.1%) I Boric Acid (0.5%) + 5.94 2278 0.676 0.789 0.907 1.077 1.255 1.541 Chlorhexidine (0.01%) J Boric Acid (0.5%) + 6.01 2354 0.689 0.789 0.899 1.089 1.246 1.461 Chlorhexidine (0.05%) K Boric Acid (0.5%) + 6.05 2368 0.854 0.955 1.072 1.245 1.369 1.576 Chlorhexidine (0.1%) 

1. An improved process for electrodeposition of a coating, the improvement comprising use of an aqueous electrodeposition bath comprising: (a) an aqueous carrier having dispersed therein (i) a film forming binder comprising an epoxy-amine adduct and (ii) a blocked polyisocyanate crosslinking agent, (b) an effective amount (an amount sufficient to retard the growth of microorganisms in the electrodeposition bath) of a biocide that is a combination of (1) at least one boron-containing compound selected from boric acid, boric acid equivalents, or combinations thereof and (2) 1,6-bis [5-(p-chlorophenyl) biguanidino] hexane (chorhexidine), wherein the pH of the electrodeposition bath is 7 or less.
 2. The process of claim 1, wherein said boron-containing compound comprises boric acid salts, boric acid esters, boron oxide, and mixtures thereof.
 3. The process of claim 1, wherein said boron-containing compound comprises boric acid.
 4. The electrodeposition bath of claim 1, wherein the amount of boron-containing compound is present in an amount sufficient to provide an amount of boron in the range of from greater than 0.3% to less than 2.0% of the total weight of the bath.
 5. The electrodeposition bath of claim1, wherein the 1,6-bis [5-(p-chlorophenyl) biguanidino] hexane is present in the bath in an amount ranging from greater than 0.01% to 0.2% of the total weight of the bath.
 6. An improved process for depositing a coating by electrodeposition, wherein the improvement comprises use of an aqueous electrodeposition bath comprising: (a) an aqueous carrier having dispersed therein (i) a film forming binder comprising an epoxy-amine adduct and (ii) a blocked polyisocyanate crosslinking agent, (b) an effective amount (an amount sufficient to retard the growth of microorganisms in the electrodeposition bath) of a biocide that is a combination of (1) at least one boron-containing compound selected from boric acid, boric acid equivalents, or combinations thereof and (2) 1,6-bis [5-(p-chlorophenyl) biguanidino]hexane (chorhexidine), wherein the pH of the electrodeposition bath is greater than
 7. 